I've heard different rule of thumbs for shutter speed. Some will say that it should be no slower than 1/60. Some will say that it should be 1/Focal length. Yet some others will say that to be absolutely sure, you should use 1/2*Focal length.
So which one is it? Also, is there a minimum speed required to shoot people?
They can all be decent guidelines. In general, ignoring the focal length doesn't make a lot of sense though since it is always going to magnify shake. A lot of other factors come in to play though and these are just rules of thumb. If you have any kind of image stabilization it will throw these off.
Also, some people are much more steady than others. Personally, I'm a trained marksman and the same support and breathing techniques that I use for shooting a gun can be adapted to shooting with a camera, so for me, I can shoot much slower shots than most people without a tripod. If you're new to photography and don't have very steady hands, then your maximum exposure time is probably going to be much lower.
Your best bet is to try it for yourself and see what results you get. No equation is going to make a substitute for experience and knowledge of how well you can hold a camera still.
A good rule of thumb is to never shoot slower than your focal length. For example, if you're shooting at 24mm don't go below 1/30 (1/20 being too slow). If you're on a 200mm, try not to shoot below 1/250. This is of course a general statement and does not take into account skill, balance, weight of camera, hand positioning or caffeine intake ;)
This info was given to me by a long-time LA Times PJ years ago in Photo 100. I have found it to be good advice in most situations.
Happy Shooting!
